And when you think about it, if they were a fossil fuel, ie made from dead animal and plant matter, how come it is found so far below the surface? Where did all teh material come from that is on top of the oil?
Oil comes from the remains of plankton and simple organisms from up to 400 million years ago. That is ample time for an immense amount of sediment to be deposited.
The pressure at depth is high so oil is squeezed toward the surface. However it becomes trapped by geological structures and pools - creating a reservoir. Analysis of individual reservoirs is required to determine how many wells and their placement is required for each reservoir.